The present invention relates to drying-induced tension variation in a paper web. More specifically, the invention relates to a method for minimizing tension variation in a paper web induced by drying of the web in a paper machine.
On-machine coating and calendering processes integrated into a paper machine have set tight constraints on tension variation in the paper web. A single weak spot in the paper web simultaneously with a tension surge can destroy the performance of the whole papermaking line. In addition, a process integrated into the machine does not include any dampening stage typical of conventional off-machine finishing processes taking place apart from the machine. In a machine roll, moisture and temperature variations can even out. Also, local stress concentrations are reduced during a long stress relaxation period.
The recent development of the wet press and dryer sections of the paper machine allows a higher web speed. The runnability of the paper machine is improved by closing all open draws and by using other new components. Due to this development, control of dryer section parameters has brought new potential. The magnitude of wet and dry straining levels can be varied widely.
The objective of this paper is to examine how moisture variation and several successive draws affect the theological properties of paper during the manufacturing process. At present, tension profile is the most important available on-line measurement technique for evaluating the theological properties of the paper web. Thus, special emphasis has been placed on understanding the significance of development of cross-direction tension profiles in a dry paper web.
In the literature, one can find several studies on the development of fiber morphology and on the rheological properties of paper during drying. See: [1] Jentzen, C. A., The effect of stress applied during drying on some of the properties of individual pulp fibers. Tappi 47 (1964) 7, s. 412-418. [2] Nanko, H., Asano, S. & Ohsawa, J., Shrinking behavior of pulp fibers during drying. Papers presented at 1991 International Paper Physics Conference. Kona, Hawaii, Sep. 22-26, 1991. TAPPI Press, Atlanta, 1991. Book 2, s. 365-373. [3] Htun, M., The influence of drying strategies on the mechanical properties of paper. Doctoral thesis, Royal Inst. Technol., Stockholm 1980, 31 s. [4] Page, D. H. & Tydeman, P. A., Physical processes occurring during the drying phase. Transactions of the symposium held at Cambridge, September 1965, Vol. 1, Ed. F. Bolam, Tech. Sect. Br. Pap. Board Makers' Assoc., London, 1966, s. 371-396. [5] Giertz, H. W. & Rodland, G., Elongation of segments—bonds in the secondary regime of the load/elongation curve. Tappi 1979 International paper physics conference. British Columbia, Sep. 17-19, 1979. Tech. Sect. Can. Pulp Pap. Assoc., Montreal, 1979, s. 129-136. [6] Schulz, J. H., The Effect of Straining During Drying on the Mechanical and Viscoelastic Behavior of Paper. Tappi, 44 (1961) 10, s. 736-744.
Most studies have compared free drying, restraint drying and wet straining. Also, the effect of removal of different water fractions from fibers on shrinkage, hornification, wrinkling etc. is well established. See: [7] Weise, U., Characterization and mechanism of changes in wood pulp fibres caused by water removal. Doctoral thesis, Helsinki University of Technology, Department of forest products, Espoo 1997, 141 s. [8] Berthold, J., Water adsorption and uptake in the fibre cell wall as affected by polar groups and structure. Doctoral thesis, Royal Inst. Technol., Stockholm 1996, 41s. [9] Laivins, G. V. & Scallan, A. M., The mechanism of hornification of wood pulps. Products of papermaking, Transactions of the tenth fundamental research symposium held at Oxford, September 1993, Vol. 2., Ed. C. F. Baker, Pira International, Surrey 1993, s. 1235-1260. [10] Maloney.
It is well known that the elastic modulus of dry paper increases by the action of wet straining, while breaking strain decreases at the same time.